1. Field of the Invention
The present invention relates to a method for smoothing a surface of a glass substrate. More specifically, the present invention relates to a method for smoothing a surface of a glass substrate having a concave defect.
The present invention also relates to a substrate for a reflective mask blank used in EUV (Extreme Ultra Violet) lithography (hereinbelow, referred to as “EUV mask blank” in Description), which is obtainable by the smoothing method, and an EUV mask blank using the substrate.
2. Discussion of Background
A mask blank used in EUV lithography (hereinbelow, referred to as “EUV mask blank” in Description) is fabricated by forming a reflective film and an absorbing layer on a super-polished substrate in this order. It is most common that the reflective film comprises a reflective multilayer film having Mo films and Si films alternately stacked therein.
If a fine irregularity exists on the surface of a substrate used for fabrication of an EUV mask blank, the irregularity has an adverse effect on the reflective film and the absorbing layer, which are formed on the substrate. For example, if a fine irregularity exists on the surface of a substrate, the periodic structure of a multilayer reflective film formed on the substrate is disordered. When a desired pattern of a mask is printed on a photosensitive organic film (a so-called photo resist film) on a Si wafer by using a an exposing system, a portion of the desired pattern may have a lack of pattern or an excessive pattern other than the desired pattern is formed in some cases. The disorder in the periodic structure of a reflective multilayer film, which is caused by an irregularity existing on a substrate, is called a phase defect, which causes an important problem. It is desired that no irregularities having a size of a certain value or above exist on a substrate.
Non-patent documents 1 and 2 describe requirements relating to a defect in an EUV mask and an EUV mask blank. The requirements for such a defect are significantly severe. Non-patent document 1 describes that a defect having a size of 50 nm or above is not permissible since the presence of such a defect on a substrate causes disorder of the structure in a reflective film to form an unexpected shape in a pattern projected onto a resist on a Si wafer. Non-patent document 1 also describes that the surface roughness of a substrate needs to be less than 0.15 nm in RMS (root-mean-square) in order to prevent the roughness of a line edge from increasing in a pattern projected onto a resist on a Si wafer. Non-patent document 2 describes that it is not permissible that a defect having a size of 25 nm or above exists on a reticle coated with a reflective film, which is used in EUV lithography. Non-patent document 3 describes what size of defect on a substrate can be printed. Non-patent document 3 describes that there is a possibility that a phase defect changes a line width of a printed image. A phase defect including a surface bump having a height of 2 nm and a FWHM (full width of half maximum) of 60 nm is a phase defect that is at the border line as to whether the phase defect can be printed or not. This document describes that a defect having such a size causes an impermissible change of 20% in line width (140 nm on a mask) with respect to a line having a width of 35 nm.
Non-Patent Document 1: SEMI, P. 37-1102 (2002), “Specification for extreme ultraviolet lithography mask substrate”
Non-Patent Document 2: SEMI, P. 38-1102 (2002), “Specification for absorbing film stacks and multilayers on extreme ultraviolet lithography mask blanks”
Non-Patent Document 3; SPIE, vol. 4889, Alan Stivers., et. al., p. 408-417 (2002), “Evaluation of the Capability of a Multibeam Confocal Inspection System for Inspection of EUVL Mask Blanks”